The problem with making your own synthetic blend is that you don't know if the different additive packages in each oil work with each other for your benefit, or if they may tend to cancel each other. Blending won't do any harm, but may give poorer results than either oil alone.
Some of the blends on the market might be some low cost group I base oil + group III...just enough group III to meet ILSAC GF-3 specs. The main purpose of most blends is for marketing.
Schaeffer's is about 75-80% Group II+ and 20-25% PAO.
Ken
[ January 09, 2004, 12:01 AM: Message edited by: Ken2 ]